Conventionally, as a method of processing binary image data (line-drawing or literal data expressed by two logic levels, in which logic "1" represents a "solid" pixel, and logic "0" represents a "vacant" pixel) of such as a literal or a line-drawing image (called a "line-drawing image" hereinafter) used in a color scanner, U.S. Application Ser. Nos. 471,869 now U.S. Pat. No. 4,553,172 or 507,719 discloses the following method.
That is, as shown in FIG. 1, binary data corresponding to a line-drawing pixel block B composed of multiple line-drawing pixels Lp (covering an area being one severalth of that of a pictorial pixel), i.e., one of several portions of the pictorial pixel are obtained in an input stage. In the case of FIG. 1, one line-drawing pixel block B, which is composed of 25 line-drawing pixels, or pixels for brevity, namely 5.times.5 pixels arranged in a matrix in the main and the sub-scanning directions, corresponds to one pictorial pixel. Then thus obtained line-drawing (binary) data are expressed in a bit signal used for expressing pictorial color separation images Y (Yellow), M (Magenta), C (Cyan) and K (Black). So when data of each color separation image are expressed in a 8 bit signal, the line-drawing image data are to be expressed in 8.times.4=32 bit signal. In the abovementioned method, the line-drawing pixel block B (in FIG. 1, it consists of 25 pixels) can be in one of the following three conditions. That is, the pixel block B can be in (a) a condition in which all pixels are transparent or white, namely containing no line-drawing images (called "vacant" hereinafter), (b) a condition in which all pixels are opaque or black, namely containing line-drawing images (called "solid" hereinafter) or (c) a condition in which some of the pixels contain line-drawing images (called "hybrid" hereinafter). In this approach, data representative of one line-drawing pixel block must be expressed for example in 32 bits, which consequently brings about a disadvantageous increase in the final quantity of stored data.
On the other hand, ruling image data are obtained by a computation as in the following way. That is, at first a scale of a ruling image pixel is computed, which must correspond to one-severalth of a pictorial pixel in the main and the sub-scanning directions. And then according to a desired arrangement, ruling image data are made and put into a run-length form to be stored in a line-drawing data file. As several scanning lines for the line-drawing image correspond to one scanning line for the pictorial image, the line-drawing data must be output in sychronization with the pictorial data. More precisely, as shown in FIG. 2, line-drawing data of the "vacant" part .circle.a and the "solid" part .circle.b of Line 1 are stored in a run-length form into the line-drawing data file. The same operation is performed for Lines 2 to 5 respectively. And the line-drawing data of Lines 1 to 5 are output in parallel simultaneously to be used for driving a recording beam, wherein of course the data are reconverted into their original forms.
In this case, although the ruling image data are condensed by being put into a run-length form, this method still contains a defect in that several of the same processing and output units are necessary as the ruling image data must be output in the abovementioned parallel way.
Incidentally, literal data can also be obtained by using pre-digitized letter font data, but the abovementioned defect is not resolved as the data are output also in the aforesaid parallel way.